Human Cytomegalovirus (hCMV) is a widespread, highly species-specific herpesvirus, causing significant morbidity and mortality in immunosuppressed or immunologically immature individuals.
Several recent reviews have analyzed hCMV biology and clinical manifestations (Landolfo S et al., 2003; Gandhi M and Khanna R, 2004; Soderberg-Naucler C, 2006a; Halwachs-Baumann G, 2006). This viral pathogen infects the majority of the population worldwide and is acquired in childhood, following contact with a bodily fluid, since the virus enters through endothelial cells and epithelial cells of the upper alimentary or respiratory systems, or through the genitourinary system. Seropositivity to hCMV is more prevalent in underdeveloped countries or in those with lower income.
Following a primary infection, hCMV can persist in specific host cells of the myeloid lineage in a latent state, replicating and disseminating in many different cell types (haematopoietic cells, epithelial cells, endothelial cells, or fibroblasts) and escaping the host immune system. In fact, even though hCMV infections are maintained under control by the immune system, total hCMV clearance is rarely achieved.
The immunocompetent host can reduce the dissemination of the virus, in particular using humoral immunity, but hCMV has developed mechanisms that allow the viral genome to remain in selected sites in a latent state, so that any situation that weakens host immune functions can lead to hCMV reactivation.
Reactivation of the virus can be associated with different stress conditions or immaturity, leading to the activation and differentiation of the hCMV-infected cells. Clinical manifestations (such as retinitis, enterocolitis, gastritis, hepatitis) can be seen following primary infection, reinfection, or reactivation. About 10% of infants are infected by the age of 6 months following transmission from their mothers via the placenta, during delivery, or by breastfeeding.
hCMV is a virus that has a linear, 230 kb, double-stranded DNA genome. The expression of the hCMV genome is controlled by a cascade of transcriptional events that leads to the synthesis of more than 200 proteins that perform a large variety of biological activities (Britt W and Mach M, 1996). The structural proteins form the virion envelope that is extremely complex and still incompletely defined. It includes glycoproteins that are homologues to structural proteins identified in other herpesviridae (gB, gH, gL, gM, and gN) and can form disulfide-linked protein complexes within the virion: gCI (including only gB), gCII (including gM and gN) and gCIII (including gH, gL, and gO). The glycoproteins gN and gM are the most abundant and, together with gH and gB, have been shown to be essential for initial interaction between the envelope of the infectious virion and the host cell, and consequently the production of infectious hCMV. For this reason, compounds targeting gB, gH, gN, or gM may inhibit hCMV infection by blocking the entry of circulating hCMV virions into cells.
Treatment of hCMV infections is difficult because there are few options. The presently available drugs that inhibit viral replication (Ganciclovir, Cidovfivir, Foscarnet, Maribavir, and others drugs under development) produce a significant clinical improvement, but suffer from poor oral bioavailability, low potency, the emergence of hCMV resistance (due to mutations in the viral targets), and dose-limiting toxicities (De Clercq E, 2003; Baldanti F and Gerna G, 2003; Gilbert C and Boivin G, 2005).
Novel means for preventing and treating hCMV infection are needed, especially for immunocompromised individuals (e.g HIV patients) and in transplantation settings. In fact, hCMV is a clinically important opportunistic pathogen in HIV patients and in organ transplant recipients, where it contributes to graft loss independently from graft rejection, resulting in morbidity and mortality. For example, a rising number of bone marrow and solid organ-transplant recipients raises the likelihood of hCMV clinical manifestations, such as hCMV retinitis, in HIV-negative patients (Wiegland T and Young L, 2006).
Thus, it is important to provide drugs for universal preemptive, prophylactic hCMV-specific treatments, for example for the prevention of hCMV disease in transplant recipients (Hebart H and Einsele H, 2004; Kalil A et al., 2005; Snydman D, 2006), in patients developing hCMV-related neuropathologies (Griffiths P, 2004) or pregnancy (Revello M and Gerna G, 2003), to prevent the vertical transmission and life-threatening hCMV infection to fetuses and neonates. In fact hCMV is the major infectious cause of birth defects (such as hearing loss, delayed development, or mental retardation) which is due to a congenital or perinatal hCMV infection transmitted by an hCMV-infected mother (Griffiths P and Walter S, 2005).
Pharmaceutical compositions against hCMV may be useful for the treatment of other, more widespread diseases (such as cardiovascular and autoimmune diseases, or some types of cancer). In fact, hCMV is considered as a possible cofactor for such diseases and is associated to mechanisms leading to cell apoptosis, differentiation, and migration. Thus, hCMV is now considered a human pathogen of growing importance, for example, for long-term complications in tumour invasiveness and immune evasion, and for autoimmune or vascular diseases such as atherosclerosis or restenosis, wherein hCMV infection may alter cellular and immunological functions (Cinatl J et al., 2004; Soderberg-Naucler C, 2006b).
An alternative way to prevent hCMV infection is vaccination, which can provide protection in an array of high-risk patient populations. However, the correlation between vaccination and the resulting immune response is not fully understood and an optimal hCMV vaccine strategy (using specific candidate antigens or live attenuated vaccines) depends on the patient population being targeted for protection. Therefore, prophylactic vaccination strategies are still under evaluation or have already failed in clinical settings (Schleiss M, 2005).
In view of the present limitations of pharmacological strategies for hCMV infections, the increasing knowledge of the host-hCMV relationship, and in particular, of the hCMV-specific immune response, makes immune-based therapies good candidates to substitute, or complement, existing strategies for the successful treatment of hCMV-associated complications (Gandhi M and Khanna R, 2004).
A possible alternative can be passive immunotherapy, consisting in the administration to individuals of pharmaceutical compositions comprising therapeutic antibodies with a defined binding specificity for a pathogenic antigen (e.g. hCMV).
This therapeutic approach has been built on the antigen-binding features of antibodies and antibody fragments directed against human or non-human therapeutic targets (Dunman P and Nesin M, 2003; Keller M and Stiehm E, 2000). Passive immunotherapy has been introduced into clinical practice, rapidly expanding the opportunities for the treatment of a wide variety of diseases (including infectious diseases, immune-mediated diseases and cancer). This approach can be particularly effective in patients whose immune system is unable to produce antibodies in the amounts and/or with the specificity required to block and/or eliminate the targeted molecule (Chatenoud L, 2005; Laffly E and Sodoyer R, 2005).
In the field of hCMV treatment, a similar approach is performed by administering intravenously human immunoglobulin preparations that are obtained by pooling human plasma with high titers of anti-CMV antibodies, and commercialized for clinical uses (under the name of Cytotect or CytoGam). However, such a therapeutic approach represents only a partially satisfactory solution for blocking hCMV infection, in particular in immuno compromised patients where potent antivirals are often co-administered (Bonaros N et al., 2004; Kocher A et al., 2003; Kruger R et al., 2003).
Obviously, purified, recombinant, human antibodies that have high affinity for antigens on the hCMV surface would represent much better drugs for passive immunization. In fact, several of the hCMV glycoproteins elicit strong host immune responses, including the production of virus-neutralizing antibodies, even though the stoichiometry of the envelope proteins is variable and may be altered to escape host immune response. This response is felt to be a key component of host immunity and represents a goal of both antibody and vaccine development.
The hCMV envelope glycoproteins B (gB) and H (gH) are targets for human CMV-neutralizing antibodies for which more detailed information are available. Sera from seropositive individuals as well as monoclonal antibodies directed against these glycoproteins inhibit HCMV infection of cell cultures in vitro. In fact, there is a correlation between anti-gB and anti-gH titers and overall neutralizing activity of convalescent sera, and a significant drop of the sera neutralizing capacity after adsorption of gB- and gH-specific antibodies. Thus, hCMV envelope glycoproteins gB and gH contain antigenic domains that induce neutralizing antibodies. (Mach M., 2006; Antibody-mediated neutralization of infectivity. In Cytomegaloviruses. Molecular Biology and Immunology. Reddehase, M. (ed.) Caister Academic Press, pp. 265-283).
Human monoclonal antibodies are preferable, due to the poor results obtained with murine monoclonal antibodies. However, the development of such human antibodies for hCMV treatment has been interrupted since no virological or clinical benefits were observed in studies that evaluated the efficacy of monoclonal antibodies, for example, in hematopoietic stem cell transplantation (Boeckh M et al., 2001), or in retinitis (Gilpin A et al., 2003).
Failure of different antibodies to demonstrate clinical benefits in large trials warrants further studies aimed at the selection of antibodies, in particular fully human monoclonal antibodies that efficiently neutralize hCMV. The treatment of CMV infections would benefit from having more potent pharmaceutical compositions comprising purified human monoclonal antibodies obtained from human cells maintained in cell culture conditions or, as recombinant proteins, from the expression of human genes coding for such antibodies in mammalian cells approved for regulatory purposes.